In many manufacturing operations, a programmable controller is used to control a plurality of different processes or devices either independently or simultaneously. Usually each device is connected through a driver circuit to a bit of an output port of the controller. Often times the number of devices and processes to be controlled exceed the number of bits available on the output port. Therefore to accommodate the additional devices or processes, (1) additional controllers may be used, (2) the controller may be replaced by one having a larger output port or (3) an interface system, which couples to the output port of the controller, is used to expand the number of devices or processes that can be controlled.
One known complex and expensive interface system representative of the state of the art includes a multiprogrammer and an interface circuit. A plurality of cards, which are insertable into the multiprogrammer, are necessary to configure the multiprogrammer and the interface circuit into a system which expands the number of devices or processes which can be controlled. In order for this system to work properly, the interface circuit must be coupled to an output port which is bus compatible with the standards established by the Institute of Electrical and Electronic Engineers (I.E.E.E.).
Consequently, there is a need for an interface system which is inexpensive and which will operate from a parallel output port of the controller.